On the dissipation process in Bi2Sr2Ca2Cu3O10 tapes and bulk samples with preferential grain orientation in zero external magnetic field

Current-voltage characteristics of Bi₂Sr₂Ca₂Cu₃O₁₀ tapes and bulk samples with preferentially oriented crystallites have been measured in zero applied magnetic field and for temperatures close to the mean-field critical-temperature,T _c0. It was shown that the power-law dependence,V=AI ^a, valid for two-dimensional systems, does not correctly describe the data, which clearly indicate the existence of a finite critical-current density,J _c. The experimentalI-V curves are fitted quite well with the model which attributes the finite critical-current density to the coupling between the CuO₂ double layers. It was found thatJ _c vanishes at a temperature value belowT _c0. This behavior can result from the occurrence of vortex fluctuation-induced layer decoupling and/or from the contribution of entropy to the vortex unbinding process.     

Effect of Cd Doping on Structure and Superconductivity in Mg0.5Cd0.5B2

The effect of Cd doping on structure and superconductivity in Mg_0.5Cd_0.5B₂ fabricated by a solid-state reaction at ambient pressure has been investigated. The resulting changes in crystal structure, superconducting transition temperature T _c and critical current density J _c are characterized by X-ray diffraction, dc magnetization, electrical resistance, and magnetic measurements. It reveals that Cd does not occupy the atomic Mg sites in the MgB₂ crystal structure, but merely reacts with Mg and forms a MgCd₃ phase. It is striking to note that although the nonsuperconducting phase MgCd₃ is as high as about 67 vol.% in Mg_0.5Cd_0.5B₂, the T _c of the doped sample drops only by about 1 K. Most important, a surprising improvement of J _c of 5.0 × 10⁵ A/cm² (5 K, 0 T) has been achieved in Mg_0.5Cd_0.5B₂. It is suggested that the improvement in J _c in Mg_0.5Cd_0.5B₂ is primarily due to pinning effects induced by MgCd₃. Also, it is thought that MgCd₃ may fill up gaps among grains in MgB₂ and produce better grain linkage, which may be another source of improvement in J _c in Mg_0.5Cd_0.5B₂.     

Reproducibility of transition temperature and critical current density of thin films of YBa2Cu3O7 on (100) LaAlO3

Submicrometer epitaxial films of YBa₂Cu₃O₇(YBCO) on (100) LaAlO₃ were made by coevaporation and furnace annealing. Samples from more than a dozen runs are used in this study. The zero resistance transition temperature (T _c) is high (89 or 90 K) if the film composition is phase pure (Ba/Y=2, Cu/Y=3) or if it is enriched in Ba and Cu. For these compositions the critical current density (J _c) at 77 K has an average value of 2×10⁵ A cm^–2, with a tendency for decreasingJ _c with increasing film thickness (0.2 to 0.8m). Variations inJ _c are not correlated with deviations from ideal stoichiometry. Steeper slopes of the resistance-temperature curves above 100 K and lower values of the room-temperature resistivity are associated with high values ofJ _c. If the film composition is enriched in Y relative to Ba and Cu,T _c decreases by several degrees.     

Scaling Behavior and Estimation of Activation Energy of Polycrystalline RuSr2GdCu2O8 Superconductor from AC Susceptibility Measurements

In this study, the measured curves of AC susceptibility (ACS) components, (T) and (T), of polycrystalline RuSr₂GdCu₂O₈ (Ru-1212) superconductor were scaled onto a single curve using the peak temperature of its imaginary part (T _p) as the scaling parameter for various AC field amplitudes from 0.5 to 24 G. The dependence of the AC magnetic field amplitude on T _p is scaled as: H _ac (1–T _p/T _c)^2.25. Similarly, the current density J _c, extracted from the AC field amplitude is also scaled as: J _c (1 – T _p/T _c)^2.25. The dependencies of T _p on frequency and AC field amplitude are also investigated and the time parameter t ₀ of the order of 10^–8 s is estimated from the dependence of T _p on frequency. The dependencies of activation energy on temperature, T, and the field amplitude, H _ac, are obtained from the Arrhenius-like semilog plot of frequency () and T _p. Such dependencies on temperature and field amplitude can be described by a scaling law of the form: U(H _ac,T) = U ₀[1 – T/T _p]H _ac ^–0.17.     

Intrinsic critical current of Ag-clad (Bi,Pb)2Sr2Ca2Cu3Oz tapes

True zero-field critical current densityJ _c of a well-characterized BPSCCO/Ag tape has been determined by means of high-resolution ac susceptibility in the temperature range 77–110 K. The resultant values (30,000 A/cm² at 77 K) agree well with the transportJ _c of the same tape. Because of a very thin BPSCCO, the coreJ _c determined from the imaginary part of the ac susceptibility is nearly the same as the zero field one. AllJ _c 's follow the same (1-T/T _c )ⁿ withn=1.45 dependence.J _c shows an approximateH ^–0.5 field dependence over the explored temperature range. Accordingly, the variations ofJ _c withT andH seem to be determined by the flux creep.     

Study of Flux-Creep in Bulk Melt-Textured YBa2Cu3Ox in the Regime of Individual Pinning

Magnetic relaxation, with magnetic fields applied parallel to the c axis, was measured in a high-quality melt-textured YBa₂Cu₃O_x bar. Zero-field-cooled magnetization-versus-time data were obtained within a regime of field and temperatures dominated by single vortex pinning. The activation pinning energy U = U _M was calculated from experimental data and compared to U = U ₀ ln(J _c /J), where U, is a constant, J _c is the critical current density, and J the current density. This expression for U obtains within the individual pinning regime. A logarithmic dependence of U _M on J was found, and the data indicated that a scaling function of U _M , g(T/T _c ) = (1–T/T _c )^1/2, was appropriate for describing the single-vortex pinning regime.     

Surface planarization and masked ion-beam structuring of YBa2Cu3O7 thin films

Surface planarization and masked ion-beam structuring (MIBS) of high-T_c superconducting (HTS) YBa₂Cu₃O_7-δ (YBCO) thin films grown by pulsed-laser deposition (PLD) method is reported. Chemical-mechanical polishing, plasma etching, and oxygen annealing of YBCO films strongly reduce the particulate density (~ 10^-2 ×) and surface roughness (~ 10^-1 ×) of as-grown PLD layers. The resistivity, critical temperature T_c ≈ 90 K and critical current density J_c (77 K) > 1 MA/cm² of films are not deteriorated by the planarization procedure. The YBCO films are modified and patterned by irradiation with He⁺ ions of 75 keV energy. Superconducting tracks patterned by MIBS without removal of HTS material and, for comparison, by wet-chemical etching show same T_c and J_c(T) values. Different micro- and nano-patterns are produced in parallel on planarized films. The size of irradiated pattern depends on the mask employed for beam shaping and features smaller than 70 nm are achieved.     

Enhancement of critical current density of YBa2Cu3O7 − δ thin films by self-assembly of Y2O3 nanoparticulates

YBa₂Cu₃O_7 − δ (YBCO) thin films, possessing high critical current density (J_c), have been synthesized by embedding a homogeneous array of Y₂O₃ non-superconducting nanoclusters/nanoparticles using a pulsed laser deposition technique. The size, interparticle spacing, and density of Y₂O₃ nanoparticles in YBCO thin films were tailored by varying the number of laser pulses in order to determine the optimum size for effective immobilization of vortices. Scanning transmission electron microscopy with atomic number contrast and X-ray diffraction techniques were used to determine the size and structure of the nanoparticles. Both techniques indicate that the Y₂O₃ particles are epitaxial with respect to the surrounding YBCO matrix. The information about pinning of vortices by the nanoparticles was obtained by investigating the behavior of critical current density as a function of temperature and applied field, which in turn determines the vortex density in the sample. The superconducting transition temperature (T_c) of YBCO films with the inclusion of nanoparticles was observed to remain almost the same or decrease marginally (1-2 K) with respect to T_c of pure YBCO films deposited under identical conditions. However, J_cs of YBCO films embedded with self-assembled nanoparticles were found to be significantly higher than that of pure YBCO films. The J_c enhancement was up to five times in high magnetic field, which is a key requirement for practical application of high-T_c materials.     

AC susceptibility for superconducting slab with regions of different critical-current densities

Based on the flux creep model, the temperature T and the amplitude of the AC field B₀ dependence of the complex AC susceptibility as a function of temperature are numerically calculated in a superconducting slab composed by three parts connected by two regions of lower critical-current density J_c which characterize the non-uniform J_c distributions in inhomogeneous superconductors. By a comparison with the analytical results of a homogeneous superconductor with a constant J_c, the numerical results show that the fundamental AC susceptibility of the composite slab is obviously different from the case of the homogeneous superconductor.     

A New Method for the Estimation of Hc2 Anisotropy in Polycrystalline MgB2 Samples

By using isothermal magnetization measurements in polycrystalline MgB₂ samples, we estimate the H ^c _c2 in the interval [0, T _c]. By combining these measurements to the estimated H ^ab _c2 from the onset of the diamagnetic transition in isofield and isothermal magnetic measurements, an estimation of the anisotropy parameter can be achieved. The H ^c _c2 values coming from high quality polycrystalline samples agrees nicely to those obtained on single crystals. Our results show a temperature variation of the (T ) = H ^ab _c2/H^c2 with (T _c) 3.     

Enhanced Electrical Transport Properties of Nano NiFe2O4-added (Bi1.6Pb0.4)Sr2Ca2Cu3O10 Superconductor

The effect of nanometer sized NiFe₂O₄ (∼15 nm) addition in Bi_1.6Pb_0.4Sr₂Ca₂Cu₃O₁₀ (Bi2223) superconductor was studied. The (Bi_1.6Pb_0.4)Sr₂Ca₂Cu₃O₁₀ superconductor was prepared through the co-precipitation method and 0.01 wt% to 0.05 wt% of nano NiFe₂O₄ was added. The critical temperature (T _c), critical current density (J _c), phase formation and microstructure were investigated. All samples showed a major Bi2223 phase with (Bi,Pb)₂Sr₂CaCu₂O₈ (Bi2212) as the minor phase. The sample with 0.01 wt% nano NiFe₂O₄ showed the highest T _c and J _c: of more than 3 orders of magnitude higher than the non-added sample at 77 K. A further nano NiFe₂O₄ addition (>0.01 wt%) leads to degradation of T _c and J _c. These results indicate that addition of an optimum amount of NiFe₂O₄ nanoparticles can effectively enhance the transport critical current density in this system.     

Weak Links and Phase Slip Centers in Superconducting MgB2 Wires

MgB₂ superconducting wires were produced by the Mg diffusion method. Scanning electron microscopy (SEM), optical microscopy, dispersive X-ray analysis (EDS), and XRD diffraction were used to study the physical structure and content of the wires. Magnetic properties (T _c ^m, H _c1, H _c2, J _c by the Bean model) were obtained with a SQUID magnetometer, and transport properties (T _c ^r , H _c2, resistivity and residual resistivity ratio) were measured using a standard four-lead configuration. The V-I characteristics of the wires close to the critical temperature showed a staircase response, which was attributed to the presence of weak links, creating phase slip centers. The origin of those weak links is discussed in relation to their formation and structure.     

Transport Properties of a Tl-2201 Film Close to the Critical Temperature: The Vortex Glass Transition

We have studied the I-V characteristics of a Tl-2201 film at zero field. In the regime in which flux creep is the dominant dissipation mechanism, the J _c -T curve is divided into two parts at a temperature T _g (about 82 K), close to the critical temperature (84 K). The I-V characteristics around T _g are well described using a flux creep model. For T>T _g , J _c /J _c (0) =0.445x(l-0.525t-0.5t ² ); for T _g , J _c /J _c (0) = 0.9x(1-0.595t-0.44t ² ). Differential resistance (dV/dI) as a function of the measuring current shows a change in curvature close to T _g . The I-V curves collapsed nicely into two branches by plotting (V/I)/|T–T _g | ^v(z-1) vs. (I/T)/|T _g –T| ^2v , indicating a current–reduced vortex glass transition.     

11B NMR Study of Pure and Lightly Carbon-Doped MgB2 Superconductors

We report a ¹¹B NMR line shape and spin-lattice relaxation rate (1/(T₁T))(1/(T_1T)) study of pure and lightly carbon-doped MgB_2-xC_x_{2-x}{\rm C}_x for x = 0,0.02x = 0,0.02, and 0.04, in the vortex state and in magnetic field of 23.5 kOe. We show that while pure MgB₂_2 exhibits the magnetic field distribution from superposition of the normal and the Abrikosov state, slight replacement of boron with carbon unveils the magnetic field distribution of the pure Abrikosov state. This indicates a considerable increase of H^c_c2H^c_{c2} with carbon doping with respect to pure MgB₂_2. The spin-lattice relaxation rate 1/(T₁T)1/(T_1T) demonstrates clearly the presence of a coherence peak right below T_cT_{\rm c} in pure MgB₂_2, followed by a typical BCS decrease on cooling. However, at temperatures lower than ?\approx 10 K strong deviation from the BCS behavior is observed, probably from residual contribution of the vortex dynamics. In the carbon-doped systems both the coherence peak and the BCS temperature dependence of 1/(T₁T)1/(T_1T) weaken, an effect attributed to the gradual shrinking of the σ hole cylinders of the Fermi surface with electron doping.     

Pinning Energy of High-Tc (Bi, Pb)2Sr2Ca2Cu3Oy

The flux pinning potential energy of silver (Ag) sheathed high-T _c Bi-2223 tapes determined from measured current densities, J(T,H), is reported in this paper. Measurements of the transport current density, J, of the Ag sheathed Bi-2223 tapes were performed with an applied magnetic field directions parallel (H‖ab) and perpendicular (H‖c) to the tape’s board surface from 0 to 3 T, at temperatures from 23 to 77 K. To obtain the pinning potential energy of the Bi-2223 superconductor, an empirical scaling of J(T,H) is introduced in this study. Under the flux dynamics, the effective pinning energy, U _eff(T,H,J), can be formulated using the measured current density, J(T,H). Furthermore, the pinning potential energies, U(T,H), for both H‖ab and H‖c were quantitatively determined by the scaling of J(T,H). The two energies, U(T,H), exhibit as a function that is inversely proportional to both temperature and logarithmic magnetic field. The magnitude of U(T,H) for H‖ab is three or more times larger than that for H‖c under the considered conditions. The U(T,H) is sensitive to the temperature, but insensitive to the magnetic field. This characteristic of insensitiveness towards magnetic field is an advantage for applications on the conditions of high current density, high magnetic field, and low temperature.     

Investigation of Gd Addition Added on Magnetic and Structural Properties of Bi1.8Pb0.35Sr1.9Ca2.1Cu3GdxOy Superconductors by ac Susceptibility

This study reports the effect of Gd addition on magnetic and structural properties of Bi_1.8Pb_0.35Sr_1.9Ca_2.1Cu₃Gd _x O _y superconductor with x=0, 0.1, 0.2, 0.3, 0.4 and 0.5 by means of ac susceptibility measurements at various ac fields (ranging from 270 to 1352 A/m) and scanning electron microscopy (SEM) images. Critical onset (T_c^on)T_{\mathrm{c}}^{\mathrm{on}}) and loss peak temperatures (T _p) were qualitatively estimated from the ac susceptibility curves. The peak temperature at zero ac-magnetic field (T _p0) and intergrain critical current densities (J _c) were theoretically calculated from the ac susceptibility plots via the critical state models. The results show that peak temperatures and critical current densities were found to decrease with increasing Gd addition. Moreover, using a self-field approximation together with J _c dependence on temperature, the characteristic length (L _c) associated with the pinning force is estimated to be approximately the same as the average grain size (R _g) of the pinning center because of the linear decrease in J _c with increasing temperature. Surface morphology and grain connectivity of the samples were also obtained to degrade with increase in the Gd addition from SEM investigations.     

Electrical conductivity and domain structure in ferroelastic Tl2SeO4

We have carried out the optical observation, electrical conductivity and ²⁰⁵Tl NMR measurements, and subsequently investigated the origin of the large conductivity above ferroelastic phase transition temperature T_c (=661 K) on the basis of the domain structure and the crystal structure. Electrical conductivity exhibits the discontinuous increase around T_c with increasing temperature and becomes approximately 5 × 10⁻³ S m⁻¹ above T_c. Moreover, from the ²⁰⁵Tl NMR measurements, it is found that mobile Tl ions exist above T_c. Furthermore, from the analysis of the domain structure based on the crystal structure in the low-temperature ferroelastic phase, it is also found that the anomalously large fluctuations of SeO₄ tetrahedrons exist above T_c. It is deduced from these results that the high electrical conductivity above T_c is caused by the mobile Tl ions closely related to the anomalously large fluctuations of SeO₄ tetrahedrons accompanied by the ferroelastic phase transition.     

Novel Metal-Organic Chemical Vapor Deposition / T1F Annealing Route to Thin Films of Tl1Ba2Ca2Cu3O9+x

Thin films of TI₁Ba₂Ca₂Cu₃O_9+x have been grown on single crystal (110) LaAlO₃ by a metal-organic chemical vapor deposition process employing Ba(hfa)₂.mep, Ca(hfa)2.tet. and Cu(dpm)₂ (hfa = hexafluoroacetylacetonate; dpm = dipivaloylmethanate; tet = tetraglyme; mep =methylethylpentaglyme) as the volatile metal sources. A subsequent phase-selective annealing procedure accomplishes thallination using TIF in a bulk oxide pellet. The resulting films are nearly phase-pure and highly oriented and contain negligible fluoride by windowless energy-dispersive x-ray measurements. The films exhibit transport measured T_c = 103 K and J_c > 10⁴ A/cm² (77 K. 0 T) and at low temperatures retain J_c > 10⁵ A/cm² (5 K, 4.5 T) as measured by magnetic hysteresis.     

Microstructures of high-Jc melt-textured YBa2Cu3O7?x/Ag superconductors

Silver has been previously added to the melt-textured YBa₂Cu₃O_7–x in order to increase the critical current density (J _c ) of these materials. However, the effect of this addition on theJ _c is presently unclear. The purpose of this study is to investigate the effect of silver on both critical current density and the microstructure of the melt-textured YBa₂Cu₃O_7–x superconductors by means of X-ray diffraction, optical polarized microscopy, and transmission electron microscopy (TEM). TheJ _c of the MTG YBCO/Ag samples is more than 10⁴A/cm² under the 5 kOe magnetic field. It has been shown that as the concentration of silver increases, the fraction of the 211 phase dispersed within the 123 matrix decreases. Therefore, theJ _c slightly decreases. These results, together with the effect of the 211 phase, dislocations, and other structure defects on flux pinning, are described in this paper.     

Comparative Study of Dense Bulk MgB2 Materials Prepared by Different Methods

We report on the results of a comparative investigation of highly dense bulk MgB₂ samples prepared by three methods: (i) hot deformation; (ii) high pressure sintering; and (iii) mechanical alloying of Mg and B powders with subsequent hot compaction. All types of samples were studied by AC susceptibility, DC magnetization, and resistivity measurements in magnetic fields up to ₀ H = 160 kOe. A small but distinct anisotropy of the upper critical field connected with some texture of MgB₂ grains was found for the hot deformed samples. The samples prepared by high pressure sintering as well as by mechanical alloying show improved superconducting properties, including high upper critical fields H _c2 (₀ H _c2 (0) 23 T), irreversibility fields H _irr which are strongly shifted towards higher values H _irr(T) 0.8 H _c2(T) and high critical current J _c (J _c = 10⁵ A/cm² at 20 K and 1 T).